Succinic acid and its derivatives are widely used as specialty chemicals for applications in polymers, foods, pharmaceuticals, and cosmetics. Furthermore, succinic acid is a valuable 4-carbon intermediate useful for the production of 1,4-butanediol, tetrahydrofuran, and gammabutyrolactone.
Although the succinate ion is a common intermediate in the metabolic pathway of several anaerobic microorganisms, there are no examples of any prior art fermentation that produces succinate in large amounts or with high yields. For example, succinate is a key intermediate for anaerobic fermentations by propionate-producing bacteria, but it is only produced in low yields and in low concentrations.
Succinate is also produced by anaerobic rumen bacteria. These bacteria include Bacteroides ruminicola (hereafter written B. rumincola) whose growth and metabolism is described by Howlett, et al., Applied Environ. Microbiol., 32, 274-283 (1976) and Bacteroides amylophilus (hereafter written B. amylophilus) whose culture and growth are described by Caldwell, et al., J. Bacteriol., 98, 668-676 (1969) and by Hamlin, et al., J. Bacteriol., 72, 548-554 (1956).
Although the rumen bacteria give higher yields of succinate than do the propionate-producing bacteria, the reported fermentations were run in very dilute solutions and gave a variety of products in generally low yields. Moreover, the rumen organisms tend to lyse after a comparatively short fermentation time, thereby, leading to unstable fermentations.
In 1961, Anderson and Ordal isolated a facultative anaerobe, Cytophaga succinicans, which produced succinate, acetate, and formate from dextrose with fixation of carbon dioxide, J. Bact., 81, 139 (1961). However, this organism produced succinate in such low concentrations that it would not be economically feasible to recover succinic acid from the fermentation medium. Similar results were observed with the Bacteroides fragilis obtained from the gastrointestinal tract, Caspari, et al., Arch. Microbiol., 135, 16-24 (1983).
In order to develop a commercially attractive process to produce succinic acid by fermentation, several important fermentation and product purification criteria need to be accomplished. The fermentation should be high yield (wt%) and produce a high product concentration using inexpensive raw materials and nutrients. Since anaerobic fermentations are run at neutral or near neutral pHs, salts of organic acids rather than the acids themselves are produced. The fermentation broth also contains cells, protein and other undesirable materials. The desired product from the process is the purified acid which can be used for specialty or commodity chemical manufacture. Hence, a high yield, economical fermentation process has to be integrated with an efficient recovery and purification process.
Electrodialysis (ED) is a well known separation process where ionized compounds are separated from non-ionized or weakly ionized compounds in aqueous solutions based on transport through ion exchange membranes in an electric field. The process has been used in a commercial scale in the chlor-alkali, desalination, metal-processing, wastewater treatment, pharmaceutical and food processing industries. Since in a fermentation broth the succinate salt is ionized, whereas the carbohydrates and proteins and amino acids are either non-ionized or weakly ionized, recovery and purification of succinate salt from the fermentation broth by electrodialysis is feasible.
Recently, several papers and patent applications have been published on recovery and purification of lactate from fermentation broths by conventional electrodialysis. However, in these examples, broth was either thoroughly cleaned to remove cells and proteins prior to electrodialysis or when broth containing cells was used the membrane fouled and led to a loss of efficiency. Prigent (6) disclosed a method for production of lactate from fermenting whey where the broth was filtered by ultrafiltration to remove the cells and then electrodialyzed. Hongo, Nomura and Iwahara (7) used a whole broth of Lactobacillus delbrueckii IFO 3534 in their electrolysis cum electrodialysis apparatus, but discovered that the efficiency was poor due to membrane fouling. These workers (8) then devised a complicated fermentation process where the cells were immobilized in alginate beads and the cell free broth was fed to the electrodialyzer. Bacterial cells are very small and to remove cells by high speed centrifugation and/or ultrafiltration is expensive both in process capital and power requirements. Cell immobilization in alginate beads is also very expensive.
It is conceivable that conventional electrodialysis, as described previously, if selective can be used to recover and concentrate succinate salt from a cell-free fermentation broth. For most specialty and commodity chemical uses of succinic acid, the acid form is required. Recent development of high efficiency bipolar membranes (9,10,11,12) for use in water-splitting electrodialysis membrane stacks, make it feasible to produce acid and base from a salt. Thus, we theorized that the desirable succinic acid and the corresponding base might be produced from the succinate salt. The base could then be recycled to the fermentation process.
The succinic acid product, after water-splitting ED, will have cationic, anionic and amino acid impurities. Ion exchange resins are capable of removing ionically charged species from solutions. Since some of the impurities in the succinic acid are ionically charged, careful selection of ion exchangers might allow removal of the impurities without removing the succinic acid itself. Cation exchangers will remove positively charged ions having a specific ionic strength. Anion exchangers will remove negatively charged ions having a specific ionic strength. Therefore, we theorized that proper selection of the types of ion exchangers, order of use, and the operating parameters may allow the removal of the succinic acid stream impurities.
For a succinic acid fermentation process to be economically attractive to produce specialty and commodity chemicals, development of a low cost fermentation and purification process is necessary. The fermentation should use low cost substrates and nutrients, the rate of fermentation should be high (high productivity), and the product concentration in the fermentation broth should be high. In addition, proper integration of an efficient and economical purification process with the fermentation is necessary.